The heparin-binding fibroblast growth factor (FGF) family consists of eight genetically distinct polypeptides which elicit receptor-mediated effects on growth, function and differentiation in most cells and tissues. The tyrosine kinase receptor family (FGFR), which together with heparan sulfate proteoglycan forms the FGFR complex, is even more diverse and consists of the products of at least 16 single site splice variations in four genes which form combinatorial variants in the extracellular, intracellular juxtamembrane and COOH-terminal intracellular domains. Expression of both individual genes of the FGF ligand and receptor family as well as combinatorial splicing is regulated and cell type-specific. Although this diversity presents great flexibility for transient regulation of multiple cellular functions, it also presents large potential for dysfunction underlying multiple pathologies. Using combined immunochemical, biochemical, molecular and cellular approaches, this continuation project will determine structure-function relationships in the extracellular and intracellular domains of FGFR isoforms. The impact of splice variations in the extracellular domain on FGF ligand-affinity and specificity and dimerization of FGFR monomers will be determined. The impact of splice variations in the intracellular kinase domain and variations in specific tyrosine phosphorylation sites on interaction with and phosphorylation of specific SH2 domain signal transduction substrates will be determined. Finally, using site-directed mutagenesis of cDNAs and their expression in bacteria, specific domains within FGF ligands that confer specificity of interaction with distinct structural domains in FGFR kinase isoforms and the active FGFR/heparan sulfate complex will be determined. These results are essential to understand the molecular basis of the multiple functions of FGF action in health and disease and the rational design of both ligand and receptor antagonists.